How to fix your dirt bike rear brake master cylinder

[Sandblaster]

How to fix your dirt bikes rear brake master cylinder.
Nothing worse then loosing your brakes as your bombing down some trail and it always seems to happen when you really need them
Most of the time, brakes fail because they have been sitting around.
We see that a lot because being in the salvage business our brakes may site around for a few years before they sell.
Then, you pull them from storage and go to box them up when you realize that they don't work...
So here is the culprit.
It's from a 96 RM125.
The calipers move easily in and out but the master cylinder plunger is stuck big time.
So here is the plan.
We will disassemble them, clean them really good, re-assemble, and bleed and see if that fixes the problem.
Believe it or not, in most cases you do not need to install new seals but hey, if your going this far, what not

Here is what we are starting with:

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As you can see it's filthy dirty and has corrosion around the plunger.
Out of all the things I can tell you to do, GET IT CLEAN is probably the most important.
We didn't do it but I highly recommend that you clean the outside really good before you take it apart.
That way you can help prevent contamination.
After you clean it externally, drain all the fluid.
The last thing you want to do is clean the master cylinder and then pump a bunch of contaminated fluid down into the caliper.
Next I like to remove the reservoir and get it out of the way.
To remove the reservoir you will need to remove the snap ring as shown below:

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They can be a little tricky so you may need to put on your reading glasses...

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Once you get it look inside.
If yours is as bad as this one its no wonder your having problems.
These should be perfectly clean inside.

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Next I remove the rubber cover along with the clevis so I can get it out of the way as well as clean it, wire wheel the threads ect.

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You will need to remove another snap ring.
Once you remove the snap ring you can pull the plunger assembly straight out the bottom.

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As you can see this plunger is filthy and rusted.
This is actually fairly common.
I like to take the plunger and wire wheel it along with all the other parts, just be careful not to damage the rubber seals or you will be buying a rebuild kit.

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Once you get it all done it should look a lot better.

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To clean the inside of the master cylinder I use a Dremel tool and a small wire wheel.
Once I get everything cleaned up I use brake cleaner to make sure all the small little particles are gone.
I also blow the brake cleaner into the small holes where the reservoir hose attaches.
If there is even a small amount of debris in these holes, your brake will not perform correctly and it may not work at all.
Once everything is ultra clean I like to put a small amount of brake fluid on the seals and then insert the plunger back into the master cylinder.

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Sometimes the snap ring is a pain to get into the correct position but it's a must for proper operation.

Last we fill the reservoir and then bleed the system with a vacuum tool.

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Put the cap on the reservoir and test the brake by moving the plunger up and down.
The brake pads should move freely and tighten.
Be careful not to get your finger stuck in the pads because they are very unforgiving.

There you have it.
Now, because we are not experts we highly recommend that you get out your shop manual and read the brake rebuild section carefully.
We may have missed something or your brakes may require some special tooling or procedure not covered here.
Once you get the brake mounted on your bike test it thoroughly BEFORE you ride it.
Thanks!
Sandblaster.